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Original Studies |
Westmead Institute for Cancer Research, University of Sydney, Westmead Hospital, Westmead, New South Wales 2145, Australia
Address all correspondence and requests for reprints to: Dr. Rosemary Balleine, Westmead Institute for Cancer Research, University of Sydney, Westmead Hospital, Westmead, New South Wales 2145, Australia. E-mail: rosemary{at}hemonc.wh.su.edu.au
Primary transcripts of the human estrogen receptor (ER) and
progesterone receptor (PR) are subject to a number of alternative
splicing events resulting in a range of variant messenger ribonucleic
acid species in receptor-positive tissues. Despite in
vitro demonstrations of a possible role for some of these
variants in hormonal sensitivity, the clinical significance of this
process is uncertain. In this study the coexpression of variant ER and
PR transcripts has been documented by RT-PCR and Southern blot analysis
in a series of receptor-positive breast tumors. In 35 ER-positive
tumors, a common profile of variant ER transcripts was present, with
all tumors containing the
2ER and
7ER,
94% containing the
4ER, and 83% containing the
5ER. In 25 of these cases, which were also PR positive,
the most highly expressed PR variants, the
4PR,
6PR, and
4/2PR, a transcript from which a
126-bp portion of PR exon 4 was deleted, were detected in over 90% of
the cases. The alternatively spliced ER variants were expressed at
higher relative levels than the PR species, which had mean levels of
expression less than 10% that of wild-type PR. The most abundant
species was the
7ER, which was present at levels ranging
from 2983% of the wild type. There was no relationship between the
level of
7ER in individual tumors and the pattern of
expression of the estrogen-responsive proteins PR and pS2. The common
profile of alternatively spliced ER and PR transcripts in breast tumors
means that this feature cannot be used as a discriminator of hormone
responsiveness or other clinical end points. Further, the low level of
expression of the majority of variant species calls into question their
potential for impacting significantly on receptor function.
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